Standard amorphous silicon electronic portal imaging devices (a-Si EPIDs) are x-ray imagers used frequently in radiotherapy that indirectly detect incident x-rays using a metal plate and phosphor screen. These detectors may also be used as two-dimensional dosimeters; however, they have a well-characterized nonwater-equivalent dosimetric response. Plastic scintillating (PS) fibers, on the other hand, have been shown to respond in a water-equivalent manner to x-rays in the energy range typically encountered during radiotherapy. In this study, the authors report on the first experimental measurements taken with a novel prototype PS a-Si EPID developed for the purpose of performing simultaneous imaging and dosimetry in radiotherapy. This prototype employs an array of PS fibers in place of the standard metal plate and phosphor screen. The imaging performance and dosimetric response of the prototype EPID were evaluated experimentally and compared to that of the standard EPID.
Clinical 6 MV photon beams were used to first measure the detector sensitivity, linearity of dose response, and pixel noise characteristics of the prototype and standard EPIDs. Second, the dosimetric response of each EPID was evaluated relative to a reference water-equivalent dosimeter by measuring the off-axis and field size response in a nontransit configuration, along with the off-axis, field size, and transmission response in a transit configuration using solid water blocks. Finally, the imaging performance of the prototype and standard EPIDs was evaluated quantitatively by using an image quality phantom to measure the contrast to noise ratio (CNR) and spatial resolution of images acquired with each detector, and qualitatively by using an anthropomorphic phantom to acquire images representative of human anatomy.
The prototype EPID's sensitivity was 0.37 times that of the standard EPID. Both EPIDs exhibited responses that were linear with delivered dose over a range of 1–100 monitor units. Over this range, the prototype and standard EPID central axis responses agreed to within 1.6%. Images taken with the prototype EPID were noisier than those taken with the standard EPID, with fractional uncertainties of 0.2% and 0.05% within the central 1 cm2, respectively. For all dosimetry measurements, the prototype EPID exhibited a near water-equivalent response whereas the standard EPID did not. The CNR and spatial resolution of images taken with the standard EPID were greater than those taken with the prototype EPID.
A prototype EPID employing an array of PS fibers has been developed and the first experimental measurements are reported. The prototype EPID demonstrated a much morewater-equivalent dose response than the standard EPID. While the imaging performance of the standard EPID was superior to that of the prototype, the prototype EPID has many design characteristics that may be optimized to improve imaging performance. This investigation demonstrates the feasibility of a new detector design for simultaneous imaging and dosimetry treatment verification in radiotherapy.
The authors would like to acknowledge funding support from the Cancer Institute NSW (Research Equipment Grant 10/REG/1-20) and Cancer Council NSW (Grant ID RG 11-06). S.J.B. would also like to thank The University of Sydney and the Institute of Medical Physics for scholarship support, as well as the Liverpool and Macarthur Cancer Therapy Centers for additional financial support. Thanks also to Rob Saunders of Nucletron for facilitating prototype design discussions with Saint-Gobain Crystals. The authors report no conflicts of interest in conducting the research.
II. METHOD AND MATERIALS
II.A. Detector design and settings
II.A.1. Standard EPID configuration
II.A.2. Prototype EPID configuration
II.B. Detector sensitivity, linearity, and pixel noise
II.C. Dose response evaluation
II.C.1. Off-axis response
II.C.2. Field size response
II.C.3. Transmission factors
II.D. Image quality evaluation
II.D.1. QC-3V phantom
II.D.2. Anthropomorphic phantom
III. RESULTS AND DISCUSSION
III.A. Detector sensitivity, linearity, and pixel noise
III.B. Dose response evaluation
III.B.1. Off-axis response
III.B.2. Field size response
III.B.3. Transmission factors
III.C. Image quality evaluation
III.C.1. QC-3V phantom
III.C.2. Anthropomorphic phantom
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